This invention relates to sensors and more particularly to a capacitive sensor having an improved sense head for detecting the presence of objects.
Capacitive sensors for detecting objects in proximity of the sensors are known in the prior art. These sensors have included a sensor head remotely connected to a capacitive loading to ground detection electronic circuit by a shielded wire having an inner conductor and an outer shield conductor such as, for example, a coaxial cable. The signal on the inner conductor is monitored by the electronic circuitry to detect the capacitive loading to ground. A low impedance ac voltage substantially identical to the ac voltage on the inner conductor is applied to the outer shield for minimizing the capacitive loading effect of the shield to inner conductor capacitance.
The sense head in its simplest form is an unguarded electrode attached to the inner conductor. For this sense head to be of practical application, a mounting is required to fix the location of the sense electrode. Problems have existed with the mounting. One mounting method included mounting the electrode in a dielectric cavity to provide a fixed separation between the sense electrode and the mounting base. With the mounting base at ground potential, a substantial capacitance between the sense electrode and ground exists; further this capacitance is subject to change with thermally induced mechanical movement or thermally induced changes in the dielectric constant of the mounting material.
To alleviate the mounting problem of the unguarded system, a guarded system was developed. The guarded system includes a conductive housing connected to a driven guard shield for supporting the dielectric material and the sense head. The conductive housing is in turn attached by means of insulating material to the grounded mounting surface.
An obvious problem attending the structure of the guarded type sensor head is that the additional parts increases the bulk of the sensor head, and the difficulty of fabricating a sensor head of a size to fit in very thin mechanisms such as, for example, in robot end effectors that work in very tight spaces. A second problem is the difficulty of making the sense electrode geometry in various shapes to accomodate special sensing applications, such as, for example, a crescent shape to provide improved resolution when detecting disk shaped objects moving transversely past the sense head. A third problem is the manufacturing expense owing to the series of nesting parts which are generally machined in cup shape and assembled surrounding the sense electrode.
Although in principle the prior art devices and the device of the present invention are substantially the same, the essential difference is the thin printed circuit board sensor head provided by the invention hereinafter described.